here's some low downs on understanding tire sizes and how you know if it'll fit or not.

Here's something interesting, you often see tire sizes starts with a P on most cars, i.e. P205/55/16, the P stands for P-metric, it means the tire is meant for a passenger vehicle, not for heavy duty use etc, there are also other letters which tells you what they are for. But we won't going into that, its unlikely any of you will try to tow 4000lbs with your E30 318.

Section width
P205/55/16, the 205 is section width of the tire, it simplly means it is 205mm wide, to convert it into inches, you simply divide it by 25.4 , which would be around 8", so if you want to fit a 275 tire on your e36, you can find out if the tires is going to fit all simplly take a ruler and measure how much space you have under the fender. HOWEVER, different companies/models have different contact width, we will go into that later on.

Sidewall Aspect Ratio
P205/55/16, the 55 is the sidewall aspect ratio, which indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall.

And you guys know the last 2 digits mean the inner diameter of the tire, or the diameter of the rims.

Internal Construction
This is not necessarily to know if the tire is going to fit or not, but it is useful to know.

A letter (R in this case) that identifies the tire's internal construction follows the two digits used to identify the aspect ratio.

P225/50R16, P225/50ZR16

The R in the P225/50R16 91S size identifies that the tire has a Radial construction in which the tire's body plies "radiate" out from the imaginary center of the wheel. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold.

If the R in the size was replaced with a D (225/50D16), it would identify that the internal tire body plies crisscross on a Diagonal and that the tire has a "bias ply" construction. Tires using this construction are for light truck and spare tire applications.

If the R in the size was replaced with a B (225/50B16), it would identify that the tire body plies not only crisscross the tire on a diagonal as before, but that they are reinforced with belts under the tread area. This type of tire construction is called "Belted." Tires using this construction are practically extinct.

I'm not going to into details about speed rating, I'm sure most of you understand what that means. If not, here's a chart to show you what each letter stands for.

Choose the right size tires
so will 215/45/17 tires fit on my E36 you said? it depends is the width of the wheel, you can't fit 215/45/17 tire on a 10" wheel at most places, simplly it is quite dangerous to seat the beads properly. And if you are not going for the mad euro look, it is simplly pointless.

Each company has spec sheet for each line of their tire models, it will tell you the following:
1) Tread depth, how much tread these tires have when brand new, most are between 10-11/32nd.
2) Tire contact patch width, its the width for the width of the area that the tire comes into contact with the road, it varies from tires to tires.
3) Tire cross section width, its the width from sidewall to sidewall before mounted on a wheel, it also varies from tires to tires.
4) Rim width, this is important, if you are not going with the euro look, follow their spec sheet, DO NOT USE ANY WHEELS THAT ARE NARROWER than minimum width, for example, if the 215/45/17 tire from Manufacture X said rim width is 7" to 8", do not use 6.5" wide wheel, it can be potentially dangerous since the wheels is a LOT narrower than the actualy cross section and may cause the sidewall to "buckle", in another word is you soften the sidewalls too much and the tire cannot perform as the manufacture intent it to be.

so remember, look for the tire spec sheet, you can find them online by searching on Google, every manufacture has them online. Even commercial truck tires.

Another main thing is speedometer accuracy, by changing your tire size/sidewall, you may affect how accurate your speedometer/odometer can read. Since you change the overall diameter it may also affect your acceleration. The rules of thumb are: if you lower your overall diameter, your acceleration will improve, but your top speed will decrease, vice versa if you increase your overall diameter.

Here's a tire size calculator so you know how much overall diameter difference there are between your new and old tires, and usually, if the difference is withing 3% you don't really have to worry about the accuracy, and most BMWs read their speed/odo from the rear wheelshttp://www.1010tires.com/TireSizeCalculator.asp